EP0509868A1 - Pneumatic servomotor - Google Patents

Abstract

The invention relates to a pneumatic brake booster comprising a casing (10) inside which there is a piston (22, 14, 102) formed of a rear tubular part (22) supporting a skirt (14 ) and which, using a rolling membrane (12), defines a front chamber (16) permanently connected to a vacuum source and a rear chamber (18) selectively connected to the front chamber (16) or to atmosphere by a valve means (136, 138, 140) actuated by a control rod (34) capable of resting, via the front face (32b) of a plunger (32), on the rear face (58a) of a reaction disc (58) integral with a push rod (56), a return spring (150) of the control rod (34) being disposed between the skirt (14) of the piston and the plunger (32), the valve means (136, 138, 140) comprising a valve (138) stiffened by an insert (118) and cooperating by an active part with a first valve seat (136) formed on the th plunger (32) and with a second valve seat (140) formed on the piston (14), the valve (138) being formed by a flexible tubular membrane (114), the active part of the valve (138) being located between the two ends (112, 116) of the flexible tubular membrane (114). According to the invention, the plunger (32) comprises a front part (126), an active part (128) movable in translation with respect to the front part (126) and a rear part (124) screwed onto the front part (126).

Description

The present invention relates to pneumatic servomotors, and more particularly of the type of those which are used to provide braking assistance for motor vehicles.

Servomotors of this type conventionally comprise a piston comprising a rear tubular part and a skirt and which, using a rolling membrane, defines a front chamber permanently connected to a vacuum source and a rear chamber selectively connected to the front chamber or to the atmosphere by a valve means actuated by a control rod capable of bearing, by means of a plunger, on one of the faces of a reaction disc secured to a push rod, a return spring of the control rod being disposed between the skirt of the piston and the plunger, the valve means comprising a valve stiffened by an insert and cooperating by an active part with a first valve seat formed on the plunger and with a second valve seat formed on the piston, the valve being formed by a flexible tubular membrane, the active part of the valve being located between the two ends of the tubular membrane full.

Such servomotors, as illustrated for example by document EP-A-0 004 477, if they have satisfactory operation, nevertheless have some drawbacks. Thus, to prevent the control rod from having too long a dead stroke, it is necessary to design the valve means in such a way that the "valve lift" between the valve and the second valve seat is as low as possible. It therefore follows that, during braking, the passage offered to atmospheric air to the rear chamber is reduced, as is the passage offered to air from the rear chamber to the front chamber. The operation of these servomotors is therefore accompanied by air suction noises, which can become annoying, especially since the structure of the piston hub, having a single radial passage towards the rear chamber and a single axial passage to the front chamber, which also induces significant turbulence in the moving air. Such servomotors also have devices for adjustment of the value of the servomotor jump, either to reduce variations due to manufacturing tolerances in mass production, or to adjust it to a particular value. These adjustment systems are often very complex, but above all they are systems internal to the actuator. The adjustment of the jump must therefore take place before the assembly of the booster according to complex processes involving specific and delicate tools to be implemented. In addition, when you want to change the value of the jump of the servomotor, either because it results from a bad adjustment, or because you want to give it another value, it is then necessary to disassemble the servomotor to make this adjustment, which is long, delicate, and an additional source of handling errors.

The present invention therefore aims to provide a servomotor whose dead travel is as small as possible, whose jump is adjustable to any desired value after the final assembly of the servomotor, from the outside, and whose operation is silent .

According to the present invention, the plunger comprises a front part, an active part which can be displaced in translation relative to the front part, and a rear part screwed onto the front part.

• La Figure 1 est une vue de côté, en coupe longitudinale, représentant la partie centrale d'un servomoteur pneumatique d'assistance au freinage de type connu, par exemple du document précité, et
• La Figure 2 est une vue de côté, en demi-coupe longitudinale, représentant la partie centrale d'un servomoteur pneumatique d'assistance au freinage réalisé conformément à la présente invention.

Other objects, characteristics and advantages will emerge from the following description of an embodiment of the present invention, given without limitation, with reference to the appended drawings in which:

• FIG. 1 is a side view, in longitudinal section, showing the central part of a pneumatic brake booster of known type, for example from the aforementioned document, and
• Figure 2 is a side view, in longitudinal half-section, showing the central part of a pneumatic brake booster produced in accordance with the present invention.

Figure 1 shows a part of a brake booster intended to be placed in the usual way between the brake pedal of a vehicle and the master cylinder controlling the hydraulic braking circuit of this vehicle. By convention, the front of the booster is called the part of the latter facing the master cylinder and the rear of the booster the part facing the brake pedal.

The booster of Figure 1 comprises an outer shell 10 in the form of a shell, having a symmetry of revolution about an axis X-X ′. Only the rear central part of this envelope 10 is shown in FIG. 1.

A flexible elastomeric rolling membrane 12, reinforced in its central part by a metal support disc 14 also called skirt, defines inside the space delimited by the envelope 10 a front chamber 16 and a rear chamber 18. The edge outer peripheral (not shown) of the membrane 12 is tightly fixed on the outer envelope 10. The inner peripheral edge of this same membrane ends in a bead received in a sealed manner in an annular groove formed on the outer peripheral surface d 'A hollow assistance piston 20 arranged along the axis XX' of the booster. This hollow piston 20 extends towards the rear in the form of a tubular part 22 which passes in leaktight fashion through the rear wall of the casing 10. The tightness of this crossing is ensured by a reinforced annular seal 24 which is fixed by a ring 26 in a tubular central part extending the rear wall of the casing 10 towards the rear.

A compression spring 28 interposed between the piston 20 and the front wall (not shown) of the outer casing 10 normally maintains the piston 20 and the skirt 14 in a rear position of rest illustrated in FIG. 1, in which the rear chamber 18 has its minimum volume and the front chamber 16 its maximum volume.

In its central part located between the tubular rear part 22 and the front part in which the membrane 12 and the skirt 14 are fixed, the piston 20 has a bore 30 in which is received in sliding a plunger 32 also having a symmetry of revolution around of the XX ′ axis. The front end of a control rod 34 of the booster, also arranged along the axis XX ′, is pivotally mounted in the plunger 32. The rear end (not shown) of this rod 34, which projects at outside of the tubular part 22 of the piston 20, is controlled directly by the vehicle brake pedal (not shown).

The annular space 36 delimited between the control rod 34 and the tubular part 22 of the piston 20 opens to the external atmosphere at the rear of the booster, for example by means of a filter with air. Forward, this same annular space can communicate with the rear chamber 18 through a radial passage 38 formed in the central part of the piston, when assistance means controlled by the plunger 32 are actuated.

Conventionally, these assistance means comprise a three-way valve comprising an annular valve 40 mounted in the tubular part of the piston and two annular valve seats 20a and 32a formed respectively on the central part of the piston 20 and on the plunger 32.

The valve 40 constitutes the front end, of smaller diameter, of a flexible elastomer sleeve, the rear end of which ends in a bead sealingly mounted inside the tubular part 22 of the piston 20. This bead is held in place by a metal cup 42, on which is supported a compression spring 44 tending to move the valve 40 forward.

The annular valve seat 32a is formed on the rear end face of the plunger 32. Similarly, the annular valve seat 20a is formed on the rear end face of the central part of the piston 20, around the seat 32a . Depending on the position of the plunger 32 inside the piston 20, this arrangement allows the valve 40 to be in constant sealing contact with at least one of the valve seats 32a and 20a under the action of the spring 44.

A second passage 46 is formed in the central part of the piston 20, approximately parallel to its axis XX ′, to make the front chamber 16 of the booster communicate with an annular chamber 48 formed around the valve 40, inside the tubular part 22 of the piston 20. When the plunger 32 occupies its rear rest position illustrated in FIG. 1, in which the valve 40 is in leaktight support on the seat 32a of the plunger 32 and separated from the seat 20a of the piston 20, the front chambers 16 and rear 18 of the booster thus communicate with one another through passage 46, annular chamber 48 and passage 38.

In a manner which is also conventional, at least one stop member 50 mounted in the central part of the piston 20 delimits the axial stroke of the plunger 32 inside the latter. The plunger 32 is normally held in the rear rest position defined by the member 50 by means of a compression spring 52 interposed between the cup 42 and a washer 54 itself resting on a shoulder formed on the control rod 34.

In its central part, the piston 20 comprises an annular front face 20b at the center of which opens the bore 30. This annular front face 20b of the piston 20 acts on a rear face 56a of a push rod 56, through a reaction disc 58 made of a deformable material such as an elastomer. More specifically, the push rod 56 and the reaction disc 58 are arranged along the axis XX ′ of the booster, in the extension of the control rod 34 and of the plunger 32. The rear surface 56a of the push rod 56 is formed on a disc-shaped plate 56b constituting the rear end of the rod 56. The plate 56b and the reaction disc 58 are capped by a cover 60 centered on the axis XX ′ of the booster and cooperating with a annular groove formed on the central part of the piston 20, around the annular front face 20b of the latter.

The operation of this known servomotor is conventional and can be described succinctly as follows.

When the booster is installed on a vehicle, the front chamber 16 communicates permanently with a source of vacuum.

Initially, the depressing of the brake pedal by the driver has the effect of equalizing the prestressing force of the spring 52 minus the prestressing force of the spring 44. During this slight displacement of the rod 34 and the plunger 32, the valve 40, under the action of the spring 44, follows the seat 32a of the plunger 32, until it comes into contact with the seat 20a of the piston; the front 16 and rear 18 chambers of the booster are then isolated from one another.

In this first phase of actuation of the booster, the force exerted on the control rod 34 does not generate any force on the push rod 56 at the outlet of the booster.

In a second phase of the actuation of the brake, the plunger 32 is moved sufficiently forward so that the valve 40 is in sealed contact with the seat 20a of the piston and begins to move away from the seat 32a of the plunger. Under these conditions, the rear chamber 18 of the booster is isolated from the front chamber 16 and enters into communication with the atmosphere. An assistance force is therefore generated which tends to move the piston 20 forwards. This movement is transmitted to the push rod 56 by the reaction disc 58.

During this second brake actuation phase, the assistance force exerted by the piston 20 does not deform the reaction disc 58 sufficiently for the latter to completely fill the space which initially separates it from the plunger 32. Consequently , the output force applied to the master cylinder by the push rod 56 increases suddenly, while the force exerted on the control rod 34 remains unchanged.

This sudden increase in the output force corresponds to what is called the jump of the booster, that is to say at the threshold beyond which the assistance force generated in the booster and exerted on the disc. reaction 58 by the piston 20 becomes sufficient for the front face 32b of the plunger to come into contact with the reaction disc 58.

Conventionally, as shown in Figure 1, the value of the jump is adjusted by the stop member 50 which determines the rear position of rest of the plunger 32 relative to the piston 20, and therefore the distance at rest between the front face 32b of the plunger and the rear face 58a of the reaction disc. It is therefore understandable that, if the jump of the booster does not have the desired value, a complete disassembly of the booster is necessary to change this stop member or the plunger or both.

During the subsequent operation of the booster, the rear chamber 18, which previously communicated with the front chamber 16 and was therefore under reduced pressure, therefore sucks air at atmospheric pressure through the valve passage of small section between the valve 40 and the plunger seat 32a. It therefore follows a significant obstacle to the passage of air, and therefore a noisy operation. In addition, the air which comes, for example, from the upper half of FIG. 1 to enter the passage 38, will have to bypass the control rod 34 and the plunger 32 to arrive at the radial passage 38, whence a turbulent flow and noisy air, added to the whistles due to the crossing of the gap between the valve 40 and the plunger seat 32a.

The same phenomena occur during brake release, when air under higher pressure is caused to pass from the chamber rear 18 towards the front chamber 16 via the radial passage 38, the gap between the valve 40 and the piston seat 20a, the annular chamber 48 around the valve 40 and the axial passage 46. These phenomena will therefore not be described in detail .

It is therefore understood that it is highly desirable to produce a servomotor whose jump value can be easily adjusted to any desired value without the need to dismantle the servomotor, and therefore from outside the servomotor, and whose operation be silent.

This object is achieved thanks to the invention, an example of an embodiment of which is shown in FIG. 2, where the elements identical to those of FIG. 1 bear the same reference numbers.

We see in Figure 2 that the piston and the plunger have been deeply modified to achieve this goal. The piston is composite, it comprises a tubular rear part 22 and a front part of revolution about the axis X-X ′ forming an intermediate piston web 102 of generally approximately frustoconical shape. On the front end of larger diameter of the web 102 is fixed, for example force-fitted, the skirt 14, on the outer peripheral edge of which is fixed the unwinding membrane 12, for example by means of openings 104 made near the edge of the skirt 14 to perfect the attachment of the material of the membrane 12. The inner peripheral edge of the skirt 14 is folded in the direction of the axis XX ′ so as to form a shoulder 106 cooperating with a shoulder 108 formed on a sleeve 110 having an annular front face 20b intended to cooperate with the reaction disc 58 as described in relation to FIG. 1.

Between the skirt 14 and the front end of larger diameter of the web 102 is tightly held a bead 112 forming the front end of a flexible tubular membrane 114 whose rear end forms a bead 116 held tightly in the piston 20, in the zone of connection of the rear tubular part 22 with the intermediate web 102, for example by means of a cylindrical metallic cup 117.

An intermediate part of the flexible tubular membrane 114 receives on its rear face an annular insert 118, so as to stiffen it in a plane perpendicular to the axis XX ′. The membrane 114 and the insert 118 have openings 120 and 122 respectively, arranged one opposite the other.

It can therefore be seen that the portion of membrane 114 rigidified by the insert 118 can move axially in the annular volume 123 situated between the rear face of the skirt 14 and the front face of the intermediate piston web 102, the skirt 14 and the web 102 being integral with one another, the web 102 itself being the forward extension of the tubular rear part 22 of the piston.

The plunger 32 is formed with a rear part 124 sliding in the bore 30 of the tubular part 22 and a front part 126 sliding inside the sleeve 110 and cooperating with the reaction disc 58. These two parts are joined by screwing one on the other. On the front part 126 slides in leaktight fashion a third part 128 of the plunger. This third part 128, or active part of the plunger, comprises a cylindrical portion 129 of internal diameter equal to the external diameter of the front part 126 of the plunger 32 and capable of sliding on it in leaktight manner. This part 129 is extended at its front end by a part extending radially outwards to form an annular part 130, then, from the outer peripheral edge of the latter, axially forwards inside of the tubular part 22 to form a cylindrical part 132, extending around the sleeve 110 and penetrating into the annular volume 123. The front end of the cylindrical part 132 is itself extended radially outwards by an annular part 134 , coming in the annular volume 123 in front of the part of the membrane 114 rigidified by the insert 118 and whose outside diameter is slightly greater than the inside diameter of the insert 118. The annular part 134 forms a first valve seat with the membrane 114 stiffened by the insert 118 forming a valve 138. The valve seat is advantageously constituted by a bulge 136, formed for example on the peripheral edge ique outside of the annular portion 134 and directed rearward, or alternatively, as shown, by a bulge formed on the front surface of the membrane 114.

A second valve seat 140 is formed on the rear face of the skirt 14, in a circle of diameter slightly smaller than the outside diameter of the insert 118. The valve seat 140 may advantageously be formed on a convex part of the rear face of the skirt 14, so that the corresponding concave part of the front face of the skirt 14 forms a housing for the compression spring 28. As before, this valve seat 140 may be formed by a bulge produced on the rear face of the skirt 14 or, as shown, by a bulge produced on the front surface of the membrane 114.

Openings 142 are made in the skirt 14 to communicate the front chamber 16 with the part of the annular volume 123 located in front of the valve 138. Likewise, openings 144 are made in the intermediate web 102 to communicate the rear chamber 18 with the part of the annular volume 123 located behind the valve 138. Finally, openings 146 are made in the rear part 124 of the plunger 32 to make the annular space 36 communicate behind the plunger 32, where atmospheric pressure prevails, with the annular volume 123, via the annular space 148 between the cylindrical part 132 of the plunger 32 and the tubular part 22 of the piston. Advantageously, it will be possible to provide, as shown, an air filter 149 at the openings 146, to avoid the intrusion of impurities in the booster.

The piston 20 is returned to its rear position of rest by the spring 28 acting on the front face of the skirt 14. It then comes into abutment against the membrane 114 in its part stiffened by the insert 118, which itself comes in abutment against the front face of the rear part of the casing 10. The second valve passage 138-140 between the valve seat 140 of the piston 20 and the valve 138 is then closed.

The plunger 32 and the control rod 34 are returned to their rear position of rest by a return spring 150, bearing on the one hand on the rear face of the part of the skirt 14 which forms the shoulder 106, and d on the other hand on the front face of the annular part 130 of the third part 128 of the plunger 32. In this position, the plunger 32 abuts, by means of the annular part 134 forming the first valve seat, against the membrane 114 in its part stiffened by the insert 118, itself already in abutment against the front face of the rear part of the casing 10, as we have seen above. The first valve passage 136-138 between the valve seat 136 of the plunger 32 and the valve 138 is then also closed.

On the other hand, the valve 138 is urged forward by a valve spring 154 bearing on the one hand on the rear face of the membrane 114 stiffened by the insert 118 forming the valve 138, and on the other hand on the front face of the bead 116 of the membrane 114 or that of the cylindrical cup 117 as shown.

Finally, the plunger 32 includes a spring 156 disposed between the rear face of a shoulder 158 formed on the front part 126 of the plunger 32 and the front face of the annular part 130 of the third part 128 of the plunger 32, so as to urge the third part 128 in separation from the front part 126 and in abutment against the rear part 124.

It can therefore be seen that a booster has been produced in accordance with the invention comprising a movable composite piston consisting of the skirt 14, equipped with the unrolling membrane 12, and integral with the intermediate web 102, itself extending towards the rear by the tubular rear part 22, this movable piston being capable of acting on the push rod 56, through the reaction disc 58, via the front annular face 20b of the sleeve 110, the latter also serving as an element guide for the plunger 32.

The valve means consist of the valve 138 formed on an intermediate part of a tubular membrane fixed at its ends to the composite movable piston, cooperating with a plunger valve seat 136 and a valve seat 140 of the composite movable piston.

The operation of this booster produced in accordance with the invention is easily deduced from the foregoing explanations. The actuator being at rest as shown in Figure 2, the front chamber 16 is in communication with a source of vacuum and is isolated from the rear chamber 18 by the valve passages 140-138 and 136-140 both of which are closed as seen above. An actuation of the control rod 34 has the effect of advancing the plunger 32 against the action of the spring 150. The valve 138 remains pressed against the valve seat 140 of the piston 20 under the action of the spring 154 , while the valve seat 136 begins to move away from the valve 138. Air at atmospheric pressure is therefore immediately admitted into the rear chamber possibly through the air filter 149, and through the openings 146 in the rear part of the plunger 32, the annular space 148 between the cylindrical part 132 of the plunger 32 and the tubular part 22 of the piston, the valve passage 136-138, the openings 120 and 122 formed in the membrane 114 and the insert 118, and finally the openings 144 in the intermediate web 102.

It is therefore clear that, in accordance with an object of the invention, a servomotor has been produced whose dead travel is as small as possible since all the operating phase which was necessary for isolating the front chambers has been eliminated. and back of each other. The only dead stroke of the booster of the invention is that which is due to the elasticity of the first valve seat 136 necessary to ensure its tightness, but it can be considered to be negligible.

We also see that, thanks to the invention, air is admitted into the rear chamber 18 through the valve passage 136-138 which is of a diameter several times larger than in a conventional servomotor as described in relation with Figure 1. It was thus possible to make valve passages 136-138 with a diameter equal to five times the diameter of a conventional valve passage.

It follows, in this example, that the passage section offered to the air is also multiplied by five, and therefore that the air flow to the rear chamber is also multiplied by five. We therefore obtain a servomotor whose operation is silent since such an enlarged passage section allows the air to circulate without causing suction noises and other whistles. It is understood that the value of five is not critical, but has only been chosen as an example.

Any other factor for enlarging the valve passage relative to a conventional servomotor may be chosen as a function of the desired result.

We have seen above that the jump of the booster occurs at this stage of operation of the booster, the air admitted into the rear chamber of the booster creating a pressure difference between the two faces of the piston, generating an assistance force which tends to move the piston 20 forward, this force being transmitted to the push rod 56 by the shoulder 106 of the skirt 14 acting on the sleeve 110, itself acting on the reaction disc 58. It then occurs a sudden increase in the force exerted by the push rod 56 until the reaction disc 58 is sufficiently deformed to fill the space which initially separates its rear face from the front face of the plunger 32.

The value of the jump of the servomotor is therefore conditioned by the axial distance at rest between the front face of the plunger and the rear face of the reaction disc. Thanks to the invention, this value can be easily adjusted to any desired value, once the servomotor has been assembled. In fact, we have seen that the plunger 32 consists of the rear part 124 screwed onto the front part 126, a spring 156 bearing on a shoulder of the latter part to urge the active part 128 on which is formed backwards. the first valve seat 136, in abutment against the second part 124. It is easily understood that, once the booster is fully assembled as shown in FIG. 2, a relative rotation of the rear 124 and front 126 parts, for example maintaining a fixed one while printing a rotation around the axis XX ′ to the other using an appropriate tool, becomes a translational movement of the front portion 126 relative to the third part 128. The coast or axial distance between the front face of the plunger 32 and the first valve seat 136. is thus varied. The plunger being at rest bearing on the valve 138 on which the skirt 14 of the piston also bears. 2 0 to form the second piston valve seat 140, and the skirt 14 comprising the shoulder 106 carried on the shoulder 108 of the sleeve 110 whose front annular face is in contact with the reaction disc 58, the relative rotation of the parts rear 124 and front 126 therefore varies the distance between the front face of the plunger 32 and the rear face of the reaction disc 58 well.

The value of the jump of the booster according to the invention can therefore be adjusted from the outside, in a simple manner by any suitable method. We could for example apply to the push rod 56 a force corresponding to the desired jump of the booster, which causes a deformation of the reaction disc 58.

It will then suffice to screw the front part 126 of the plunger into the rear part 124 to advance the front face 32b of the plunger 32 until it comes into contact with the deformed part of the rear surface 58a of the reaction disc, contact which can be detected by any means, for example using a strain gauge placed on the control rod 34. By then releasing the force exerted on the push rod 56, there will be a servomotor adjusted to the desired jump value. The actuator can also be operated by actuating it by its control rod 34. Force sensors on the control rod 34 and the push rod 56 will make it possible to draw the diagram of the output force as a function of the input force, and therefore to adjust the jump of the booster to the desired value.

We have therefore produced in accordance with the invention, a booster whose jump value can be adjusted to any desired value from outside the booster, and therefore without the need to dismantle it. This actuator has an extremely reduced dead travel, and operates without noise.

It will be understood that the phenomenon of reduction of the operating noise of the booster also occurs during brake release. In fact, when the force applied to the control rod decreases, the latter moves back and drives the plunger 32 in its movement. In doing so, the valve seat 136 of the plunger 32 returns to contact with the valve 138, the valve seat 140 still being in contact with the valve 138. When the plunger 32 continues to reverse, the valve seat 136 of the plunger then causes the valve 138 to move away from the valve seat 140. The air contained in the rear chamber 18 is then drawn into the front chamber through the openings 144 in the intermediate web 102, the openings 122 and 120 in the membrane 114 and the insert 118, the valve passage 138-140 and finally the openings 142 in the skirt 14. We therefore see here again that the air is caused to flow through the valve passage 138-140, of a diameter much greater than conventional valve passages, and therefore of greater surface area which allows a higher flow rate, from which it also results silent operation in phase d e brake release.

The invention also makes it possible to significantly reduce the operating noise of the booster. We have seen in fact that, during braking, the air flows through the openings 146, the annular space 148, the valve passage 136-138, and the openings 120, 122 and 144, and during the brake release, by the openings 144, 122 and 120, the valve passage 138-140 and the openings 142. The particular design of the booster according to the invention makes it possible to provide that the openings 142, 120, 122, 144 and 146 are in equal number, regularly distributed around the axis XX ′, and so that their centers are in the same plane, as shown in Figure 2. In this way, the air masses set in motion during the operation of the booster of the present invention will have a speed whose components will be contained only in a plane, for example that of Figure 2. In other words, the flow of air in the booster is perfectly symmetrical around axis XX ′ in all operating cases, that is to say that all turbulence is eliminated, as well as the noises resulting therefrom.

It is therefore clear that a pneumatic servomotor has been produced in accordance with the present invention in which the particular arrangement of the valve and plunger means allows easy adjustment of the value of the jump by operating from outside the fully assembled servomotor, the servo motor with extremely reduced dead travel and quiet operation. Of course, the invention is not limited to the embodiment which has been described by way of example, but is capable of receiving numerous variants which will appear to those skilled in the art. Thus, for example, the unwinding membrane and the flexible tubular membrane on which the valve is formed can be formed in one piece. Likewise, the invention may be applied to servo motors in tandem or with an additional chamber.

Claims (11)

Pneumatic brake booster comprising a casing (10) inside which is a piston (22, 14, 102) formed by a rear tubular part (22) supporting a skirt (14) and which, at using a rolling membrane (12), defines a front chamber (16) permanently connected to a vacuum source and a rear chamber (18) selectively connected to the front chamber (16) or to the atmosphere by a valve means (136, 138, 140) actuated by a control rod (34) capable of pressing, via the front face (32b) of a plunger (32), on the rear face (58a ) a reaction disc (58) integral with a push rod (56), a return spring (150) of the control rod (34) being arranged between the skirt (14) of the piston and the plunger ( 32), the valve means (136, 138, 140) comprising a valve (138) stiffened by an insert (118) and cooperating by an active part with a first valve seat (136) formed on r the plunger (32) and with a second valve seat (140) formed on the piston (14), the valve (138) being formed by a flexible tubular membrane (114), the active part of the valve (138) being located between the two ends (112, 116) of the flexible tubular membrane (114), characterized in that the plunger (32) has a front part (126), an active part (128) movable in translation relative to the front part (126) and a rear part (124) screwed onto the front part (126). Actuator according to claim 1, characterized in that the active part (128) slides in leaktight manner on the front part (126) and is in abutment against the rear part (124), a spring (156) being disposed between the front parts (126) and active (128) of the plunger (32). Booster according to claim 2, characterized in that the front part (126) comprises the front face (32b) of the plunger (32) which is capable of bearing on the rear face (58a) of the reaction disc (58), and in that the first valve seat (136) is formed on the active part (128). Actuator according to claim 1, characterized in that the piston (22, 14, 102) further comprises an intermediate piston web (102) defining with the skirt (14) of the piston an annular volume (123), and in that the first (136) and second (140) valve seats and the valve (138) are arranged in the annular volume (123), the second valve seat (140) being formed on the skirt (14) of the piston. Booster according to claim 4, characterized in that openings (142) are made in the skirt (14) to make the front chamber (16) communicate with the annular volume (123), and openings (144) are made in the intermediate web (102) for communicating the rear chamber (18) with the annular volume (123). Actuator according to claim 5, characterized in that the valve (138) has openings (120) located in its active part between the first valve seat (136) and the second valve seat (140), and allowing communication between the front chamber (16) and the rear chamber (18). Booster according to any one of the preceding claims, characterized in that openings (146) are made in the plunger (32) to make the annular volume (123) communicate with the atmosphere. Booster according to claim 7, characterized in that the openings (146, 120, 142, 144) made in the plunger (32), the valve (138), the skirt (14) and the web (102) are regularly distributed around of the axis of symmetry of the servomotor, are in equal number and have their centers located in the same plane. Actuator according to one of the preceding claims, characterized in that the spring (150) for returning the control rod (34) is arranged between the skirt (14) of the piston and the active part (128) of the plunger (32) . Actuator according to one of the preceding claims, characterized in that a valve spring (154) biases the active part of the valve (138) forwards, this valve spring (154) being concentric with the spring (150) of reminder of the control rod (34) and of larger diameter. Method for adjusting the value of the jump of a servomotor according to any one of the preceding claims, characterized in that it comprises the following steps: - A force is applied to the push rod (56) corresponding to the value of the desired jump of the booster. - The front part (126) of the plunger (32) is screwed into the rear part (124) to advance the front face (32b) of the plunger. - The contact between the front face (32b) of the plunger and the rear face (58a) of the reaction disc (58) is detected. - The force applied to the push rod (58) is released.

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